This invention relates to a process for recycling a spent sulfuric acid, for example, exhausted from the stage of producing titanium dioxide, by regeneration.
Titanium dioxide has been used in a large quantity as a white pigment in paint industries and as a coloring material in various fields. As a process for producing titanium dioxide, a so-called "sulfate process" is most commonly known. In this process, ilmenite or titanium slag is milled into particles with particle sizes of not more than 200 mesh sieve pass, and then treated with concentrated sulfuric acid to give titanium sulfate solution. While adjusting the concentration by the addition of water or dilute sulfuric acid, scrap iron is added thereto in order to prevent the precipitation of iron ions contained as impurities, thereby reducing trivalent iron ions to bivalent iron ions. This is followed by allowing the solution to cool, to separate out ferrous sulfate. After removal of the separated substances, hydrolysis of the titanium sulfate solution gives titanium hydroxide, which is then filtered, washed, and finally calcinated to obtain titanium dioxide.
In these stages for producing titanium dioxide, a large quantity of spent sulfuric acid is chiefly exhausted in the stage of separation between the solid and liquid in titanium hydroxide. Conventionally, concerning the treatment Process for the spent sulfuric acid, it was buried as gypsum or neutralized and then thrown out to the ocean. However, environmental problems have recently been given a great deal of attention. Moreover, problems of securing a site to be filled up with the gypsum, and of expensive treatment costs have arisen. How to deal with them is important.
In order to deal with these problems, processes for recovering sulfuric acid from the spent sulfuric acid by separating of impurities from spent sulfuric acid have been suggested. For example, a process in which the spent sulfuric acid is concentrated as it is (Japanese Patent Publication Sho 46-5568, etc.) and a process in which the spent sulfuric acid is treated in a combination of a vacuum crystallization stage and a concentration stage are mentioned. Due to the deposition of a large quantity of metal sulfates, these process are, however, problematic in that a desired highly concentrated sulfuric acid cannot be obtained.
More recently, Japanese Patent Laid-Open Hei 3-80103 and Hei 3-88718 disclose processes for removing impurities with a solvent extraction method. Nevertheless, since a strongly acidic sulfuric acid solution is treated by means of a solvent extraction as it is in these processes, the actual efficiency for extracting iron ions is not high as described in these patent applications. Moreover, the iron content thus recovered has low purity. Furthermore, these applications suggest the recovery of iron ions using methyl isobutyl ketone, but it is impossible to directly extract trivalent iron ion with a neutral extracting agent, such as methyl isobutyl ketone, in which case the iron is extracted as a complex such as a chloride complex. Also, in the case where an acidic extracting agent is utilized in order to directly extract trivalent iron ions from the sulfuric acid solution, multi-stage extraction is necessarily involved due to its low extraction ratio.